1. Field of the Invention
The present invention relates to a semiconductor device having a circuit formed using a transistor and a manufacturing method thereof. For example, the present invention relates to an electronic device on which an electro-optical device typified by a liquid crystal display device is mounted as a component.
In this specification, a semiconductor device means all types of devices that can function by utilizing semiconductor characteristics, and an electro-optic device, a semiconductor circuit, and electronic equipment are all semiconductor devices.
2. Description of the Related Art
As a liquid crystal display device, an active matrix liquid crystal display device in which pixel electrodes are arranged in matrix and transistors are used as switching elements connected to respective pixel electrodes in order to obtain a high-quality image, has attracted attention.
An active matrix liquid crystal display device, in which a transistor formed using a metal oxide for a channel formation region is used as a switching element connected to each pixel electrode, has already been known (see Patent Document 1 and Patent Document 2).
It is known that an active matrix liquid crystal display device is classified into two major types: transmissive type and reflective type.
In the transmissive liquid crystal display device, a backlight such as a cold cathode fluorescent lamp is used and an optical modulation operation of liquid crystal is utilized to choose one between the two states: a state in which light from the backlight passes through liquid crystal to be output to the outside of the liquid crystal display device and a state in which light is not output, whereby bright and dark images are displayed; further, image display is performed in combination of these.
Since the backlight is utilized in the transmissive liquid crystal display device, it is difficult to recognize display in the environment with strong external light, for example, outdoors.
In the reflective liquid crystal display device, the optical modulation operation of liquid crystal is utilized to choose between the two states: a state in which external light, that is, incident light is transmitted through liquid crystal and reflected on a pixel electrode to be output to the outside of the device and a state in which incident light is not output to the outside of the device, whereby bright and dark images are displayed; further, image display is performed in combination of them.
Compared to the transmissive liquid crystal display device, the reflective liquid crystal display device has the advantage of low power consumption since the backlight is not used; therefore, a demand for the reflective liquid crystal display device as a portable information terminal has increased.
Since external light is utilized in the reflective liquid crystal display device, the reflective liquid crystal display device is suited for image display in the environment with strong external light, for example, outdoors. On the other hand, it is difficult to recognize display when the liquid crystal display device is used in a dim environment, that is, in the environment with weak external light.